Allow increasing the software PWM frequency.

The first commit allows increasing the software PWM frequency using SOFT_PWM_SCALE. The second commit adds information into Configuration.h to explain how to use that to get the fan to spin at a low frequency which is not annoying. I've tested that with FAN_SOFT_PWM and SOFT_PWM_SCALE=2 the fan is driven at a frequency that is not annoying (without SOFT_PWM_SCALE it's driven at a very low frequency which is annoying, to me at least).